An investigation was made of ionic transport in the 0.2[xNa2O(1−x)Rb2O]0.8B2O3 mixed-alkali system, with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, in the glassy and undercooled liquid states by means of impedance spectroscopy and tracer diffusion experiments. The glass transition temperature, obtained by differential scanning calorimetry, exhibited a minimum as a function of composition. The compositional dependence of the conductivity below Tg also exhibited a minimum. The direct-current σT values obeyed the Arrhenius law below and above Tg. The glass transition appeared as a kink in the Arrhenius plot of σT. Below the glass-transition temperature, the onset frequency of conductivity dispersion exhibited an Arrhenius-like temperature dependence. According to so-called Summerfield scaling, the activation enthalpies of σT and the onset frequency were expected to be the same. This was observed only for single-alkali compositions. The activation enthalpies of σT as a function of composition exhibited a classical mixed-alkali maximum. However, the activation enthalpies of the onset frequencies as a function of composition exhibited an almost constant behavior; in contrast to the expected from Summerfield scaling. The tracer diffusion measurements revealed a major difference in the diffusion of 86Rb and 22Na in mixed-alkali glasses. A diffusivity cross-over of tracer diffusion coefficients of 22Na and 86Rb occurred near to x = 0.2 (figure 2). By comparing tracer and conductivity diffusivities, a Haven ratio was deduced which exhibited a maximum near to the conductivity minimum composition.

Ionic Conduction, Diffusion and Glass Transition in 0.2[xNa2O(1-x)Rb2O]▪0.8B2O3. A.W.Imre, S.Voss, H.Mehrer: Journal of Non-Crystalline Solids, 2004, 333[3], 231-9